The demonstration of an apparent cure for HIV after allogeneic hematopoietic stem cell transplantation of a leukemia patient in Berlin offers an important proof-of-principle for AIDS gene therapy. However, inefficient transfer of anti-HIV genes has limited success in AIDS gene therapy clinical trials. We previously developed foamy virus (FV) anti-HIV vectors that potently inhibit HIV replication and efficiently transduce human repopulating cells in a mouse xenotransplant model. Here we will develop improved combinatorial FV vectors designed to inhibit HIV escape and to be safer in preparation for using FV vectors in the clinic. Our proposal directly address two critical challenges for AIDS gene therapy, 1) to develop combinations of transgenes that potently inhibit HIV replication and escape, and 2) to improve the safety of HSC gene therapy and better understand safety in terms of potential clonal expansion and activation of proto-oncogenes. FV vectors offer several advantages for AIDS gene therapy including a distinct and potentially safer integration profile and the ability to efficiently deliver anti-HIV transgenes that interfere with HIV-based lentiviral vector titers. We will develop improved, safer FV anti-HIV vectors and directly compare their safety to lentiviral vectors which are currently being used in clinical trials. We will also explor ways to make FV vectors safer by modifying the integration profile to direct integration away from genes including proto-oncogenes. We will evaluate safety in vivo in human repopulating cells using an established xenograft model we have extensive experience with. Our proposal takes advantage of several unique properties of FV vectors and builds upon our published data showing FV vectors effectively deliver anti-HIV transgenes that interfere with HIV-based lentiviral vectors.